21.
A:Acquiring new ideas while reviewing old knowledge
B: by re-animating the Old can gain knowledge of the New
C:If a man keeps cherishing his old knowledge, so as continually to be acquiring new.
Meaning:Generally, people may understand this idiom from two angles. One, reviewing old knowledge and acquiring new ideas operate in parallel: One can gain new inspiration while reviewing old knowledge. Two, reviewing old knowledge is the prerequisite for acquiring new ideas: One who does not review old knowledge will not be able to acquire new ideas. In this process, the new will replace the old, and the outdated will be abandoned.